On Thu, Jun 26, 2014 at 03:56:32PM +0100, Mel Gorman wrote:
> On Thu, Jun 26, 2014 at 10:37:38AM -0400, Johannes Weiner wrote:
> > On Thu, Jun 26, 2014 at 09:43:14AM +0100, Mel Gorman wrote:
> > > On Wed, Jun 25, 2014 at 04:35:28PM -0700, Andrew Morton wrote:
> > > > On Wed, 25 Jun 2014 08:58:48 +0100 Mel Gorman <mgorman@xxxxxxx> wrote:
> > > >
> > > > > @@ -325,7 +321,14 @@ static unsigned long zone_dirty_limit(struct zone *zone)
> > > > > */
> > > > > bool zone_dirty_ok(struct zone *zone)
> > > > > {
> > > > > - unsigned long limit = zone_dirty_limit(zone);
> > > > > + unsigned long limit = zone->dirty_limit_cached;
> > > > > + struct task_struct *tsk = current;
> > > > > +
> > > > > + if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk)) {
> > > > > + limit = zone_dirty_limit(zone);
> > > > > + zone->dirty_limit_cached = limit;
> > > > > + limit += limit / 4;
> > > > > + }
> > > >
> > > > Could we get a comment in here explaining what we're doing and why
> > > > PF_LESS_THROTTLE and rt_task control whether we do it?
> > > >
> > >
> > > /*
> > > * The dirty limits are lifted by 1/4 for PF_LESS_THROTTLE (ie. nfsd)
> > > * and real-time tasks to prioritise their allocations.
> > > * PF_LESS_THROTTLE tasks may be cleaning memory and rt tasks may be
> > > * blocking tasks that can clean pages.
> > > */
> > >
> > > That's fairly weak though. It would also seem reasonable to just delete
> > > this check and allow PF_LESS_THROTTLE and rt_tasks to fall into the slow
> > > path if dirty pages are already fairly distributed between zones.
> > > Johannes, any objection to that limit raising logic being deleted?
> >
> > I copied that over from global_dirty_limits() such that the big
> > picture and the per-zone picture have the same view - otherwise these
> > tasks fall back to first fit zone allocations before global limits
> > start throttling dirtiers and waking up the flushers. This increases
> > the probability of reclaim running into dirty pages.
> >
> > Would you remove it from global_dirty_limits() as well?
> >
> > On that note, I don't really understand why global_dirty_limits()
> > raises the *background* limit for less-throttle/rt tasks, shouldn't it
> > only raise the dirty limit? Sure, the throttle point is somewhere
> > between the two limits, but we don't really want to defer waking up
> > the flushers for them.
>
> All of which is fair enough and is something that should be examined on
> a rainy day (shouldn't take too long in Ireland). I'm not going to touch
> it within this series though. It's outside the scope of what I'm trying
> to do here -- restore performance of tiobench and bonnie++ to as close to
> 3.0 levels as possible. The series is tripping up enough on the fair zone
> and CFQ aspects as it is without increasing the scope :(
You asked to remove it, I'm just asking follow-up questions ;-)
I agree that this is out-of-scope for your patches and it probably
should be left alone for now. However, I do like your comment and
wouldn't mind including it in this change.
Would everybody be okay with that?
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